Structural health monitoring to assess the integrity of civil infrastructure, aerospace components, and military aircraft components is critical for the estimation of the optimal service life of these structures and prevention of catastrophic failure. Crack and strain sensing are some of the aspects of structural health monitoring. Catastrophic failure may be caused by strains or displacement beyond allowable (or design permitted) levels, and failure is often preceded by the formation of cracks that are caused by strain.
Strain sensing is one of the most critical aspects of structural health monitoring and non-destructive evaluation. There is a demand for robust strain and displacement sensors that are light-weight, cost-effective, and easy to install. Strain sensors that can measure considerable strains over large areas of structures are needed. Traditional resistive metal foil strain gauge sensors, though low-cost and easy to install, have limitations, such as the need for a physical connection of the sensing element with the interrogation unit. Fiber-optic-based sensors have also received considerable attention since they are very compact and can provide very high resolution. However, they have inherent problems such as fragility and susceptibility to damage. In both these types of strain sensors, the physical connection between the sensor and the interrogation unit is also prone to corrosion and fracture. In addition, wireless monitoring of strains in the absence of a clear line of sight and/or physical connection to the structure of interest is impossible using conventional resistive or capacitive strain gauges. Conventional strain gauges often have active elements that deform when a load is applied. The performance of these strain sensors tend to deteriorate/drift over time since the fatigue life and mechanical properties of the metallic element determines the ultimate performance.
Recently, wireless sensors have been the subject of tremendous interest. U.S. Patent Application Publication No. 2007/0186677 A1, incorporated by reference herein in its entirety, discloses a non-contact radio-frequency (RF) strain sensor that uses a planar LC (inductor-capacitor combination) circuit element for detecting the resonant frequency change caused by structural deformation proportional to the applied stress. However, this strain sensor contains metallic sensing elements that physically deform on application of the load. The long-term performance of these strain gauges again depends on the fatigue life of the metallic sensing elements, which can typically undergo only a few thousands to tens of thousands of cycles before failure or deterioration in performance. Unfortunately, strain sensors with deformable metallic sensing elements may fail too soon to be suitable for long-term sensing applications, such as monitoring bridges, towers, aircraft components, and the like. In addition, premature failure of deformable metallic sensing elements may not be distinguishable from failure of the structure being monitored.